The complement system is a major player in innate immunity and an effector arm of the humoral immune response. Through natural Abs, lectins and the alternative pathway (AP), especially the AP's feedback loop, the complement system activates on microbes and altered self. Injured, apoptotic and necrotic cells, accumulations of debris, and microbial pathogens are targets. As we age, lipids deposit in blood vessel walls (atherosclerosis), urate in joints (gout), amyloid proteins in the brain (Alzheimer's disease - AD) and lipofuscin pigments (drusen) in the retina (age-related macular degeneration - AMD). These body wastes or garbage become substrates for complement activation, leading to chronic inflammation. Thus, regulation of the complement system, particularly the amplification loop, is critical to immune homeostasis and to prevent undesirable activation in vital organs. The goal of this grant proposal is to build on prior contributions relating to interactions of the complement's key C3b fragment with receptors and regulators and to further explore its role as a nidus for assembling the feedback loop by: 1) assessing C3b interactions with its regulators and receptors, with a goal of characterizing novel heterozygous mutations in C3 that predispose to atypical hemolytic uremic syndrome (aHUS) and on a polymorphism in C3 associated with age-related macular degeneration and renal transplant survival; 2) further defining the structure and function of complement receptor type one (CR1; CD35), including analyzing recently identified mutations associated with human disease; and 3) employing a newly generated animal model, the Crry-single knockout (SKO) mouse, and the Crry mouse to examine complement regulation in vivo with a focus on homeostasis of the AP's feedback loop and to assess models of human disease in which the AP mediates pathologic consequences. An underlying hypothesis for proposed experiments is that the AP is continuously turning over on cells and thereby serves as a surveillance system for foreign agents and altered self.